1160 Part VII / Development and the Emergence of Behavior
Figure 47–3 Dendritic branching develops in a series of
steps. The outgrowth of dendrites involves the formation
of elaborate branches from which spines develop. Certain
branches and spines are later pruned to achieve the mature
pattern of dendrite arborization. (Image of spines at right repro-
duced, with permission, from Stefan W. Hell.)
分支初始 生长 生成刺
停止
/修剪
be classified. Cerebellar Purkinje cells can be distin-
guished from granule cells, spinal motor neurons, and
hippocampal pyramidal neurons simply by looking
at the pattern of their dendrites. These variations are
critical for the distinct functions of different neuronal
types. For example, the size of a dendritic arbor and
the density of its branches are main determinants of
the number of synapses it receives.
How is dendritic pattern established? Neurons must
have intrinsic information about their shape because the
patterns in tissue culture are strikingly reminiscent of
those in vivo (Figure 47–4). The transcriptional programs
that specify neuronal subtype (Chapter 46) presumably
also encode information about neuronal shape. In both
invertebrates and vertebrates, some transcription fac-
tors are selectively expressed by specific neuronal types
and appear to be devoted to controlling the size, shape,
and complexity of their dendritic arbors. They do so
by coordinating the expression of downstream genes,
including those encoding components of the cytoskel-
etal apparatus and membrane proteins that mediate
interactions with neighboring cells.
A second mechanism for establishing the pattern
of dendritic arbors is the recognition of one dendrite by
others of the same cell. In some neurons, dendrites are
spaced evenly with respect to each other, an arrange-
ment that allows them to sample inputs efficiently
without major gaps or clumps (Figure 47–5A). In
many cases, this process, called self-avoidance, occurs
through a mechanism in which branches belonging to
the same neuron repel each other. Several cell-surface
adhesion molecules have now been found that medi-
ate self-avoidance by interacting in a way that results
in repulsion (Figure 47–5D). Although it seems coun-
terintuitive that an adhesive interaction between adja-
cent membranes would lead to repulsion rather than
attachment, the consequences of most intercellular
interactions are determined by the signaling they initi-
ate rather than by adhesion per se, as we will see later
in this chapter.
The dendrites of neighboring neurons also provide
cues. In many cases, the dendrites of a particular neu-
ron type cover a surface with minimal overlap, a spac-
ing pattern called tiling (Figure 47–5B). The tiling of
dendrites is conceptually related to self-avoidance, but
in tiling, the inhibitory dendritic interactions are among
neurons of a particular type, whereas in self-avoidance,
they are among sibling dendrites of a single neuron.
Tiling allows each class of neuron to receive informa-
tion from the entire surface or area it innervates. Tiling
of a region by the dendrites of one class of neuron also
avoids the confusion that could arise if the dendrites of
many different neurons occupied the same area.
A particularly interesting situation is one in which
dendrites engage in self-avoidance but synapse on the
dendrites of other cells of the same type. In this situa-
tion, dendrites face the challenging task of distinguish-
ing nominally identical dendrites from dendrites of